Aquariums for tropical fish generally comprise a water filled tank, an aerator and a heater provided with a thermostat to maintain the water in the tank at a predetermined temperature beneficial to the fish. In most low cost installations, the heater comprises an electrical resistance element enclosed by a protective waterproof tube immersed in the tank. Although such a simple heater is satisfactory for the types of tropical fish which can thrive over a rather wide range of temperature, this type of heater creates a large temperature gradient in the tank which can cause the rapid demise of several species of tropical fish which can survive only within a very narrow temperature range. It is well known that the water in the immediate vicinity of an immersion heater is quite warm whereas the water in the regions of the tank remote from the heater may be intolerably colder.
Circulation of the water in the tank improves the situation by reducing the temperature gradient but is insufficient in the case of certain species of fish.
The practice of providing a filter compartment in a hood which constitutes a cover for the aquarium tank and circulating tank water through the filter compartment for purification and aeration is desirable and well known. In U.S. Pat. No. 3,892,199, Huyler discloses his contribution toward the solution of the problem of temperature gradients in aquarium tanks by providing the conventional immersion heater in the filter compartment of the hood rather than in the tank proper. The increase circulation of the heated water reduces the temperature gradients. However, owing to the limited heat transfer capacity of conventional electrical resistance immersion heaters, the flow rate of water through the filter compartment must be low and the circulating water temperature elevated considerably to effect close regulation of the water temperature in the tank. A substantial temperature differential therefore exists between the heated water in the filter compartment and the water in the tank. The dimensions of the tank in which the overly heated water is returned from the filter compartment may be unsuitable for certain fish. Huyler's aquarium does not comprise a cooling unit.
Although a number of so-called universal aquariums are being marketed which comprise both heating and cooling means so that the water temperature can be adjusted to accommodate either tropical or cold water fish, most if not all of these aquariums suffer from severe temperature gradient problems since the heating and cooling elements are locally disposed and there is insufficient circulation in the tanks to reduce the temperature gradients to acceptable levels. In systems in which cooling is produced by a refrigeration system employing a refrigerant fluid such as freon, even slight refrigerant leakage into the tank water has been found to have disastrous consequences.